Exit Costs - Schedule of exit cost liability (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Restructuring Reserve [Roll Forward]
Balance, beginning of period	$ 53	$ 25
Charges	4	43
Paid	(41)	(7)
Adjustments		4
Balance, end of period	16	65
Severance and Termination Benefits
Restructuring Reserve [Roll Forward]
Balance, beginning of period	43	22
Charges	1	29
Paid	(30)	(1)
Adjustments		4
Balance, end of period	14	54
Facility Exit Costs
Restructuring Reserve [Roll Forward]
Balance, beginning of period	2	0
Charges	0	6
Paid	(1)	(6)
Adjustments		0
Balance, end of period	1	0
Contract Termination & Other Costs
Restructuring Reserve [Roll Forward]
Balance, beginning of period	8	3
Charges	3	8
Paid	(10)	0
Adjustments		0
Balance, end of period	$ 1	$ 11